1. Field of the Invention
This invention relates to a substrate for information-recording media.
2. Prior art
In information-recording media according to optical recording system, etc., the substrate on which a recording layer is laminated has a fine concave/convex-formed groove. This fine concave/convex-formed groove means concentric grooves or a spiral groove for tracking of a laser head, digitalized information pits, and the like. The substrate having such a fine concave/convex-formed groove is formed, e.g. by injection-molding a thermoplastic resin such as polymethyl methacrylate, polycarbonate, or the like.
Such a substrate having a fine concave/convex-formed groove is also formed by a method which comprises coating a transparent flat base plate with a UV ray-curable monomer to form a coating having a uniform thickness, bringing a die plate having a predetermined concave/convex form into contact therewith under pressure, irradiating the coating with UV ray from the transparent base plate side to cure the UV ray curable monomer, and then separating the die plate (this method is called a photopolymer method, and will be abbreviated as 2P method hereinafter). A variety of UV ray-curable monomer compositions have been hitherto proposed for a 2P method. For example, Japanese Unexamined Patent Publication No. 62-264459 discloses a monomer composition composed mainly of a (meth)acrylate having a bisphenol A skeleton.
Further, a substrate having a concave/convex-formed groove is also produced by a method of cast-molding a liquid monomer composition in a mold which is provided with a predetermined concave/convex form on one surface thereof. As a liquid monomer composition useful for this method, Japanese Unexamined Patent Publication No. 63-86710 dislcoses a monomer composition composed mainly of an epoxy acrylate and an alicyclic di(meth)acrylate such as bis(meth)acryloxymethyltricyclo[5,2,1,0.sup.2.6 ] decane, or the like.
However, the polymethyl methacrylate and polycarbonate used in the injection-molding method have defects in that they are poor thermal dimensional stability and are liable to cause a change such as warpage during storage for a long period of time. And, a polycarbonate substrate having a predetermined concave/convex-formed groove, produced by the injection-molding method, has a defect of a large birefringence.
Further, the UV ray-curable monomer composition composed mainly of a (meth)acrylate having a bisphenol A skeleton, useful for the 2P method and disclosed in Japanese Unexamined Patent Publication No. 62-264459, has the following defects. Its polymerizability with UV ray is unsatisfactory, and it does not give hardness and heat resistance sufficient to satisfactorily endure deformation resulting from heat and longterm storage. It also has a defect of poor replication accuracy.
The monomer composition composed mainly of an epoxy (meth)acrylate, useful for the cast-molding method and described in Japanese Unexamined Patent Publication No. 63-86710, has advantageous characteristics in that it is excellent in moldability and available at a low cost and has low birefringence. However, this monomer composition still has the following defects. That is, a polymer of this composition is that which is produced by polymerization of an epoxy (meth)acrylate as an essential monomer and the polymer has a low surface hardness. Further, the polymer has a hydroxyl group. Hence, the polymer has high water absorptivity, and the resultant substrate has poor dimensional stability. And, when a recording layer is laminated on the substrate, it is liable to be corroded. For these reasons, the resultant recording medium has a defect of poor reliability for long-term storage of recorded information.
Furthermore, after the compositions described in the above Publications are cured by UV ray or cast-polymerized, it is difficult to separate the resultant polymer from a die plate. And, when stamping operation in the 2P method or the castmolding is repeated, the cured or polymerized polymer builds up on the die plate, and the replication accuracy is deteriorated.